Aircraft: CRJ700
Phase of flight: Parked
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown

I Mechanic X was about to service an aircraft with [engine] oil when the Crew Lead asked me to help a co-worker in the Deferral Action of a hydraulic Shut-Off Valve (SOV). I approached my fellow Mechanic Y; asking what needed to be done. Mechanic Y told me to take off the hydraulic pump from the left engine while she took the parts necessary for the deferral out of the 'Fly Away' kit. Mechanic Y instructed me to install the Blank-Off Plate in the mount [on the Engine Accessory Case] where the hydraulic pump was installed and to secure it along with Cannon plug. After I installed this plate and capped the [hydraulic] quick disconnect outlets; I asked if she needed any further assistance. Mechanic Y replied that all she needed to do was to place the hydraulic pump in the 'Fly-Away' kit. I proceeded to put my tools and toolbox away. With four minutes left before clock-out time; she approached me asking to sign a Job Card for the task which I was unaware that existed. I signed the blocks required; but failed to see the part where it mentioned the Spline Plug being installed before the plate. It was determined that the error was made by an emergency landing that the aircraft had to make at ZZZ1 airport due to the [engine oil] leakage. This is to the best of my knowledge lack of communication between co-workers. I assisted my co-worker by following her instructions; she had the paperwork and failed to inform me about the Job Card; also failed to provide the parts needed to perform the task. I failed to pay the necessary attention at the end of the day to the Job Card when filling [out] the [sign-off] blocks. Lack of knowledge and experience was also a factor. I've been employed for several months in this company and have never worked on the engine section of these [CRJ-700] aircraft. Also Mechanic Y explained to me that she has never performed this procedure [either]. Some of these mistakes might have been caused by stress and fatigue because it was at the end of the night and my co-worker expressed to me that these plane had to be at the gate [in a half hour]. [Recommend] better communication between mechanics when working as a team; especially if one joins another in the middle of the task; pay more time and attention to Job Cards when completing them even under stress. CRJ-700 aircraft en route to ZZZ1; crew declared an emergency landing in ZZZ1 due to loss of oil on the Number 1 Engine. Once aircraft landed ZZZ1 Maintenance Crew removed Blank-off Plate from the Number 1 Engine Accessory Gear Box (AGB) and noticed no Spline Plug assembly was installed. [Contributing factor was] Number 1 Engine Hydraulic Firewall Shut-Off Valve (SOV) would not close once Fire Switch was pressed. Notified Supervisor; Supervisor informed me there was no Hydraulic Shut-Off Valve [in stock] and the HYD SOV would have to be deferred. Number 1 Hydraulic Firewall SOV MEL 29-11-8 and Number 1 Hydraulic Engine Driven Pump (EDP) MEL 29-11-1 per Job Card 29-XX-YYYY Engine Driven Pump Deactivation. This was approximately before dawn; the airplane was to be at the gate within an hour. My Co- worker; Mechanic X; was asked to help me defer the Number 1 Hydraulic (Hyd) EDP. We printed Job Card 29-XX-YYYY for the Engine Driven Pump Deactivation. Opened Fly-Away kit. Got EDP deferral items listed from the Fly-Away kit. Removed Number 1 Hydraulic EDP Spline from Accessory Gear Box (AGB); flexible hoses. Capped all lines and placed Blank-off Plate on Accessory Gearbox. Placed Number 1 Hydraulic EDP; Spline from AGB and flexible hoses in Fly-Away Kit. Closed Number 1 Engine Cowls. Performed Engine Run for Leak Check. No leaks noted. Mechanic X and I were under stress; knowing the aircraft was late to the gate. Got to work [later] that night and was told that the Spline Plug Assembly was not installed on the Number 1 Engine Accessory Gearbox; thus causing a oil leak and an emergency landing. My Co-Worker; Mechanic X; and I didn't realize that we had skipped a Maintenance Step in Step Two of the Job Card. The following morning I didn't perform a High Power Engine Run when checked for leaks. Mechanic X did open cowlings and Inspector verified no leaks. Mechanic X closed cowlings. The following day; to clarify my previous statement; I did perform an Engine Run and Leak Check; not realizing a High Power Run was required. I was also involved in numerous tasks at the same time. [I was] trying to get aircraft to gate.[Recommend] the Job Card should be a RII Card due to the oil loss of the engine and the high risk involved with this task. Step Two needs to be broke down in more steps. Not so many Critical Items in one step; makes it more likely for someone to miss a critical step. I would also like to see Buy-Back [second set of eyes verification] steps for the critical items. Two Line Aircraft Maintenance Technicians (AMTs) report about a company CRJ-700 aircraft that had to divert in flight due to oil quantity loss on Number 1 Engine. They had previously removed the Engine Driven Hydraulic Pump (EDP) on the Accessory Gear Box (AGB) and had been 'pushing' to meet a morning departure schedule.